Fasteners are used to form joints in e.g. different assembly processes, wherein power tools may be used to tighten such fasteners. In general it is desirable to increase the speed and accuracy in the tightening of fasteners. Another important aspect is to improve the ergonomics during operation of the power tool.
Previously, fasteners have usually been tightened to a predetermined torque without any consideration to the energy used during the process of tightening. In order to increase the tightening accuracy the speed had to be decreased during the final step of tightening after snug. Thereby the reaction forces exerted on an operator may be relatively high.
To reduce this problem, WO 2009/011633 A1 discloses a regulator for a power tool wherein the fastener of a joint is tightened to a predetermined target torque. The regulator is arranged to calculate the amount of energy which is needed to reach the predetermined target torque. Thus a high speed may be used during tightening, thereby reducing the reaction forces exerted on an operator of the tool.
However, during tightening of fasteners there may be variations in friction between different joints, which may affect the torque needed to tighten the joint. Thus, when controlling the tightening based on a target torque, there may be variations in the clamp force due to these friction variations.
Further to this, when using high rotational speeds to provide the rotational energy needed for ergonomically favourable tightening of fasteners, heat may be generated in an interface between rotating and stationary parts of the joint. Due to this, the coefficient of friction in the interface may be affected. Thus, when controlling the torque to reach a predetermined target torque there may be a variation in the final angle depending on geometrical factors of the joint, in combination with such variations in friction. Thus the resulting clamp force, being a very important quality factor of the joint, may vary based on these factors.
Variations in friction in the interface may e.g. be reduced by reducing the rotational speed during tightening, but this is disadvantageous since it decreases the tightening speed and may result in the operator being subjected to higher reaction forces.